1. Field of the Invention
The present invention relates to a method of speedily manufacturing a printing master using a thermosensitive stencil paper which is capable of yielding high quality images, without decreasing the durability of the members of a printing machine.
2. Discussion of Background
Conventionally, a printing system using a stencil paper is widely utilized because of the advantage of convenience. In this kind of printing system, a thermosensitive stencil paper is prepared by attaching a thermoplastic resin film to a porous substrate with an ink-permeability property. As the thermosensitive stencil paper thus prepared is brought into pressure contact with a thermal head with the application of pressure to the thermosensitive stencil paper using a pressure-application roller, signals are applied to the thermal head. The thermoplastic resin film is partially melted in accordance with the signals, thereby forming perforations in the thermosensitive resin film imagewise corresponding to the signals. Then, a printing ink applied to the thermosensitive stencil paper from the porous substrate side permeates through the porous substrate to be ready for printing images on an image-receiving medium such as a sheet of paper.
However, when the thermal energy is applied to the thermosensitive stencil paper comprising the porous substrate and the thermoplastic resin film attached to the porous substrate to form the perforations therein, the thermal energy required to perforate a portion of the thermoplastic resin film where the substrate is attached via an adhesive agent is larger than the thermal energy required to perforate the other portion of the thermoplastic resin film which is not supported by the substrate, that is, corresponding to a pore of the substrate. Therefore, there is the problem that a perforation formed in the thermoplastic resin film at a position where the substrate is substantially attached to the thermoplastic resin film may become small, or the perforations cannot be formed perfectly. As a result, clear images cannot be obtained on the image-receiving medium because of unevenness of the perforations formed in the thermosensitive stencil paper.
To prevent the formation of uneven perforations, it is proposed to supply an excessive amount of thermal energy to the thermal head for the perforation of the thermosensitive stencil paper. However, this curtails the life of the thermal head, and causes the burnout in the thermal head to induce the occurrence of abnormal images.
Furthermore, another problem of the thermosensitive stencil paper comprising the porous substrate and the thermoplastic resin layer attached thereto is that the printing ink cannot smoothly permeate through the porous substrate such as Japanese paper. Therefore, uniform images cannot be obtained on the image-receiving medium.
There is proposed a method of printing images on the image-receiving medium using as a printing master a thermosensitive stencil paper substantially consisting of a thermoplastic resin film. The method of manufacturing a printing master using a thermosensitive stencil paper consisting of a thermoplastic resin film is conventionally known as disclosed in Japanese Laid-Open Patent Applications 53-49519, 54-33117, 3-45719, 3-45720 and 62-282983. In the conventional manufacturing methods of the printing master as proposed in the above-mentioned applications, one surface of the thermosensitive stencil paper is brought into contact with a heating element of the thermal head, with the thermosensitive stencil paper being pressed from the opposite surface thereof toward the heating element using a pressure-application platen roller.
In this case, however, a part of the thermal energy supplied to the thermosensitive stencil paper by the heating element of the thermal head is caused to escape to the pressure-application platen roller when the thermal energy is applied to the thermosensitive stencil paper consisting of the thermoplastic resin film which is in pressure contact with the pressure-application platen roller. Consequently, a sufficient amount of thermal energy required to perforate the thermosensitive stencil paper is not supplied to the thermosensitive stencil paper due to the above-mentioned thermal loss.
To solve the aforementioned problem of the thermal loss, the inventors of the present invention have proposed a method of increasing the heat-insulating properties of the pressure-application platen roller by employing a material with a low thermal conductivity for the pressure-application platen roller, as disclosed in Japanese Patent Application 4-61339.
However low the thermal conductivity of the material is used for the pressure-application platen roller, the thermal loss is not evitable in practice because the thermosensitive stencil paper is brought into pressure contact with the platen roller. This causes some failure in the perforation of the thermosensitive stencil paper.
Furthermore, when a printing master is prepared using the thermosensitive stencil paper consisting of the thermoplastic resin film under the circumstances of low humidity, there is the problem that the thermoplastic resin film is electrostatically charged. As a result, the thermoplastic resin film is electrostatically attached to a member of the printing machine, so that a satisfactory printing master cannot be obtained.
It is difficult to meet both of the requirements for the manufacturing method of a printing master, that is, to produce the printing master speedily without decreasing the durability of the members of the printing machine, and to obtain the printing master capable of yielding high quality images.